The present invention relates to Fe-Ni alloy and method for producing the same, and more particularly relates to improvement in property of Fe-Ni alloy suited for production of lead frames used for multiple pin type integrated circuits (IC).
With recent development in the field of large scale integration circuits (LSI) and super LSI circuits, large size silicon chips are also increasingly used in these circuits and increase in size of these silicon chips is inevitably accompanied with increased heat generation in the circuits. When there is a great gap in degree of thermal expansion, between a silicon chip and a lead frame, such increased heat generation tends to pose thermal stress on the silicon chip, thereby causing breakage of the silicon chip development of cracks in the structure of the silicon chip. For these reasons, it is nowadays intensively required to make the degree of thermal expansion of a lead frame close to that of a silicon chip which is to be combined therewith, in particular in the case of lead frames used for LSI circuits and super LSI circuits.
In order to meet such a requirement, it is proposed to use an Fe-Ni alloy of low thermal expansion. For example, Japanese Patent Opening Sho. 55-119156 proposes an alloy called 42 Alloy containing 42% by weight of Ni, and Japanese Patent Opening Sho. 59-198741 discloses an alloy called Koval which contains 29% by weight of Ni and 13% by weight of Co. Another example is Koval containing 29% by weight of Ni and 17% of Co.
In production of lead frames for IC, there is a recent general trend for increase in number of pins to be planted to one lead frame. This increase in number of pins inevitably causes corresponding decrease in width of the inner lead. Conventionally, the width of an inner lead is generally in a range from 0.3 to 0.5 mm. Whereas the recent increased number of pins allow an inner lead to have a width of only 0.15 to 0.2 mm. Reduced width of the inner lead directly connects to significant lowering in its mechanical strength which tends to cause undesirable deformation of the inner lead during transportation and/or working in production. So, in addition to the above-described closeness in degree of thermal expansion, high mechanical strength of lead frames is also strongly required in practice.
A large gap in degree of thermal expansion incurs another problem. That is, during assemblage of lead frames and silicon chips in formation of a circuit, interspaces are apt to be developed between the lead frames and sealing resin due to thermal hysteresis of the lead frames and presence of such interspaces often induces malfunction of the circuit during usage. From this point of view, a lead frame is required to have good bond with sealing resin.